Disinfection of aqueous solutions

ABSTRACT

Solutions of sugars obtained during the processing of foodstuffs can be subject to bacterial contamination. In view of the possible legislation to prevent the use of existing disinfectants such as formaldehyde in these conditions, alternative treatments are needed. Effective disinfection of sugar solutions can be achieved employing a peracetic acid solution containing a high mole ratio of hydrogen peroxide to peracetic acid, such as from 18:1 to about 54:1 in combination with a second peracetic acid solution.

The present invention relates to disinfection and more particularly tothe disinfection of aqueous solutions produced during food processingoperations or like solutions containing a substantial concentration ofnutrients for bacteria.

During the course of industrially processing foodstuffs, in a number ofindustries there are produced aqueous solutions of for example sugars orlike materials capable of acting as nutrients for bacteria, includinginter alia Lactobacilli and Thermophilic Bacilli. For example, duringthe production of sucrose from sugar beet, the sliced beet solids arecontacted with an aqueous solution for a lengthy period at elevatedtemperatures in order to extract the sugars into solution. Bacteria areinevitably introduced into the process on the surface of the sugar beet.Thus, the contact period represents an excellent opportunity for thebacteria to multiply; at less elevated temperatures the Lactobacilli canthrive and at the more elevated temperatures the Thermophilic Bacillican thrive, thereby forming in situ lactic acid and/or other unpleasantor even toxic contaminants. The sugar solutions are subsequentlysubjected to purification and crystallisation steps. The sugar industryis fully aware of these potential problems and currently introduce arange of biocides in order to counteract them. These biocides includedithiocarbamates and formaldehyde. Whilst their use has been regarded aseffective, questions have been raised as to whether they should bepermitted for use in food processing. A further problem with the use offormaldehyde is that it can impart a colouration to the sugar, thusreducing its value and/or increasing the washing amount of washing ofthe sugar required which increases processing times and can also resultin increased loss of sugar. Accordingly, it is desirable to locate analternative disinfectant system.

One of the areas in the process to produce sucrose from sugar beet intowhich it is particularly desirable to introduce a biocide is thediffusers, these being the part of the plant where the chopped andwashed sugar beets are contacted with extracting liquors To extract thesucrose. The biocide added to this area is known hereinafter as "Dsolution". Another area into which it is desirable to introduce abiocide is the pressed pulp water recycle system, in which part of theliquor that is extracted from the beets is separated from the beet andrecycled back to the diffusers. The biocide added to this area is knownhereinafter as "PWC solution".

One class of compounds that have been proposed for use as a disinfectantcomprises peroxycarboxylic acids, including peracetic acid. It has beenused or proposed to be used as a disinfectant for the sugar processingindustry in a paper by Rolf Nystrand in Zuckerind. 110 (1985) Nr 8pp693-698 entitled "Disinfectants in Beet Sugar Extraction". However,the treatment regime suggested by Nystrand only comprises the use of asingle peracetic acid solution having a high mole ratio of hydrogenperoxide to peracetic acid. It has been found in the course of studiesleading to the present invention that The use of two different peraceticacid solutions dosed in separate locations gives good, cost effectivecontrol of bacterial populations.

According to the present invention there is provided a process fordisinfecting aqueous solutions of sugars or like solutions obtainedduring food processing and containing a significant amount of nutrientfor bacteria, characterised in that there is introduced into the pressedpulp water recycle an effective amount of a peracetic acid solutioncomprising a substantial molar excess of hydrogen peroxide relative tothe peracetic acid, and that there is introduced into central fractionof the diffusers an effective amount of a peracetic acid solution thatdoes not comprise a substantial molar excess of hydrogen peroxiderelative to the peracetic acid.

The process of the present invention can be carried out most simply byintroducing the compositions into the process liquors at the desiredprocess stages in amounts at suitably timed intervals.

In the PWC solution, provided that a substantial molar excess ofhydrogen peroxide over peracetic acid is employed, the precise choice ofthe composition is at the discretion of the user. It is desirable toselect a mole ratio of H₂ O₂ :PAA of at least about 12:1 and in practicethe mole ratio is normally not higher than about 120:1. In somepreferred embodiments the mole ratio is selected in the region of about18:1 to about 54:1. Although in theory the peracetic acid concentrationcould be varied through quite a wide range of concentrations, inpractice a concentration of at least 0.5% w/w is preferred to minimisethe overall volume of peracid composition for transportation and/orstorage. Usually, the peracetic acid concentration of up to about 5% w/wis selected, and for convenience and ease of manufacture, theconcentration is often from about 2% to about 3% w/w. The hydrogenperoxide in such compositions is often selected advantageously withinthe range of from about 15% to about 50% w/w.

In especially convenient embodiments, the PWC solutions for use in thepresent invention can be made by reacting a concentrated hydrogenperoxide solution, often selected from solutions containing from 30 to65% w/w hydrogen peroxide, and particularly a solution containingnominally 35% w/w with a minor amount of acetic acid or anhydride, suchas in a mole ratio of peroxide to acetic acid of about 10:1 to about30:1 and thereafter permitting the mixture to reach equilibrium. A smallamount of a customary stabiliser and/or a strong acid catalyst or acombination of catalyst and stabilisers can be incorporated, includingsulphuric acid and an organic phosphonic acid such asethylenehydroxy-diphosphonic acid typically in an amount of up to about1 or 1.5% w/w and/or an aromatic hydroxyacid such as dipicolinic acidtypically in an amount of up to about 0.5% w/w. The temperature formanufacture of the composition is at the discretion of the producer, andis usually selected in the range of at least about 10° C., taking intoaccount line rate at which it is desired to obtain product from theproduction unit and whether suitable safety provisions are incorporatedin the unit.

The concentration of peracetic acid in the D solution can be selectedfrom a wide range of concentrations, but is often in the range of fromabout 0.5% to about 40% w/w, and most often between about 4% and about20% w/w. The concentration of hydrogen peroxide in the D solution isoften selected from about 5% to about 30% w/w, but in any event, themole ratio of hydrogen peroxide to peracetic acid in the D solution isOften selected to be less than about 10:1, and most often less thanabout 5:1. It will be readily apparent to one skilled in the art that alow mole ratio of hydrogen peroxide to peracetic acid can be achieved byemploying a distilled grade of peracetic acid. In the most preferredembodiments, the concentration of peracetic acid is in the range of fromabout 10 to about 15% w/w, and the concentration of hydrogen peroxide isin the range of from about 15 to about 25% w/w.

The D solution can be prepared in any of the methods known in the art,which generally comprise reacting acetic acid or acetic anhydridesolution with hydrogen peroxide Solution, optionally at elevatedtemperature and in the presence of a strong acid catalyst, together withany desired stabilisers, such as dipicolinic acid and/or., an organicphosphonic acid such as ethylenehydroxy-diphosphonic acid.

The preferred amount of the PWC solution to introduce into the pressedpulp water circuit will naturally depend upon a number of factors, suchthe levels and frequency of recontamination that occur, the strains ofBacilli which are present and the operating conditions in the foodprocessing process. In general, it is preferred to conduct a series ofranging trials to establish the approximate minimum amount ofcompositions that should be used. In many circumstances encountered todate, PWC solution is introduced into the process liquors or likesolutions to provide a peracid concentration up to about 100 ppm, andpreferably it is selected in the range of at least 5 ppm and often up toabout 50 ppm, ie preferably from about 6.5×10⁻⁵ M to about 6.5×10⁻ M.The D solution is introduced into the diffuser to provide a peracidconcentration in the liquors up to about 500 ppm, and preferably it isselected in the range of at least 25 ppm and often up to about 350 ppm.

The invention process can be carried out over a wide range of operatingtemperatures, from ambient operating temperatures, which may be as lowas 5° C. up to about 90° C. Consequently, the invention process is wellsuited to incorporation in conventional processes for extracting sugarsfrom sugar beet. In such processes, sugar beet roots are washed, sliced,and contacted with extracting steam/water. In all processes variations,a substantial fraction of the sugars are extracted under controlledtemperature and pH conditions in a continuously operated diffuser,generally conducted with the macerated beet passing in a counter-currentfashion to the extracting liquor. In such processes, a temperaturegradient is conventional, ranging from about 40-50° C. up to about75/80° C. In some variations, a prescalder is employed for the initialcontact, which in which the operating temperature often averages about40° C. The extracting liquors are typically recirculated to at leastsome extent between stages in the diffuser, and the overall retentiontime of liquor in the diffuser is often several hours during which anybacteria which had survived the initial shock from contact withperacetic acid could multiply in the absence of residual biocide orbiostat.

The PWC solutions are dosed into the portion of the diffuser liquorscomprising the recycled liquors from the presses, most preferably afterthis liquor has been screened to remove any fine particulate matter.

The D solutions are introduced into the central fraction of thediffusers. In many practical instances, the fraction comprisesapproximately one third of the length of the diffusers, i.e. measuringfrom the liquor outlet, the D solution is dosed at a location not lessthan one third, and not more than two thirds, of the length of thediffuser. Preferably, the dosing is located in a position such that theeffective lifetime of the peracetic acid is not less than to the timefor the liquor to flow from the dosing location to the outlet. Theeffective lifetime of peracetic acid is the time taken for the peraceticacid concentration to reduce to a concentration at which it issubstantially biocidally inactive.

If desired, a peracetic acid composition can additionally be introducedinto the aqueous pulp in the pre-scalder, but in many cases, this willnot be necessary.

By the use of the process according to the present invention, it ispossible to control the growth of both Lactobacilli and ThermophilicBacilli, which tend to thrive at different stages of the sugarextraction process, using the same (i.e. peracetic acid-containing)disinfectant. This simplifies the number of different treatment agentsthat need be employed in the process.

The process according to the present invention can also result in theproduction of sugar having an increased whiteness before washingcompared with the situation where certain alternative disinfectionregimes are employed, thereby reducing the amount of washing required toproduce sugar of the desired whiteness. For, good whiteness it isdesirable to employ peracetic acid concentrations of at least 5 ppm inthe pressed pulp water circuit and at least 25 ppm in the diffuser.

Having described the invention in general terms, the effectiveness ofspecific embodiments thereof will be demonstrated by the Examples below.

EXAMPLE 1

The trial was carried out on a sugar beet processing line.

A solution of peracetic acid comprising 3% w/w peracetic acid and 30%w/w hydrogen peroxide was continuously dosed into the pressed pulp watercircuit after the liquor had been screened to remove particulate matter.The concentration of peracetic acid employed was 11 ppm. A secondsolution of peracetic acid, commercially available from Solvay InteroxLtd under their Trade Mark PROXITANE comprising 12% w/w peracetic acidand 20% w/w hydrogen peroxide, was shock dosed at a concentration of 210ppm peracetic acid into the diffusers, and thereafter maintained at aconcentration of 130 ppm peracetic acid by dosing for 10 minutes everythree hours. The second peracetic acid solution Was dosed into theseventeenth bay of a diffuser comprising 34 bays in total.

The dosing into the pressed pulp water circuit resulted in an average ofa 3 log reduction in the microbial contamination of this circuit. Thelactic acid concentration in the raw sugar from the diffuser wascontrolled to below 100 ppm lactic acid.

EXAMPLE 2

The procedure of Example 1 was followed, except that the peracetic acidsolution dosed into the pressed water circuit was dosed on a one houron, one hour off basis.

Both of these treatment regimes were found to give effective control ofthe bacterial population in the process liquors, and gave acceptably lowconversions of sucrose to lactic acid and acceptable productcolouration.

We claim:
 1. In a process of recovering sugar from sugar beet whichcomprises:contacting sugar beet with water in a diffuser to produce beetpulp and an aqueous sugar solution, pressing the beet pulp to producepressed beet pulp and pressed pulp water, recycling at least a portionof the pressed pulp water to the diffuser, and introducing adisinfecting amount of a disinfectant into the sugar solution, theimprovement in*which the step of introducing a disinfectant into thesugar solution comprises: introducing into the recycled portion of thepressed pulp water a disinfecting amount of a peracetic acid solutionhaving a mole ratio of hydrogen peroxide to peracetic acid of at least12:1; and introducing into the aqueous sugar solution in the diffuser adisinfecting amount of a peracetic acid solution having a mole ratio ofhydrogen peroxide to peracetic acid of less than 10:1.
 2. A processaccording to claim 1, wherein the peracetic acid solution introducedinto the pressed pulp water has a mole ratio of hydrogenperoxide:peracetic acid of from about 12:1 to about 120:1.
 3. A processaccording to claim 1, wherein the peracetic acid solution introducedinto the pressed pulp water has a mole ratio of hydrogenperoxide:peracetic acid of from about 18:1 to about 54:1.
 4. A processaccording to claim 1, wherein the peracetic acid solution introducedinto the diffuser has a mole ratio of hydrogen peroxide:peracetic acidless than about 5:1.
 5. A process according to claim 1, wherein theconcentration of peracetic acid in the solution introduced into thepressed pulp water is from about 0.5% w/w to about 5% w/w.
 6. A processaccording to claim 1, wherein the concentration of peracetic acid in thesolution introduced into the pressed pulp water is from about 2% w/w toabout 3% w/w.
 7. A process according to claim 1, wherein theconcentration of peracetic acid in the solution introduced into thediffuser is from about 0.5% w/w to about 40% w/w.
 8. A process accordingto claim 1, wherein the concentration of peracetic acid in the solutionintroduced into the diffuser is from about 4% w/w to about 20% w/w.
 9. Aprocess according to claim 1, wherein the concentration of peracid inthe pressed pulp water circuit is up to about 100 ppm.
 10. A processaccording to claim 1, wherein the concentration of peracid in thepressed pulp water circuit is from about 5 ppm to about 50 ppm.
 11. Aprocess according to claim 1, wherein the concentration of peracid inthe aqueous sugar solution in the diffuser is up to about 500 ppm.
 12. Aprocess according to claim 1, wherein the concentration of peracid inthe aqueous sugar solution in the diffuser is from about 25 ppm to about350 ppm.
 13. In a process of recovering sugar from sugar beet whichcomprises contacting sugar beet with water to produce beet pulp and anaqueous sugar solution, and introducing a disinfecting amount of adisinfectant into the sugar solution,the improvement wherein saidintroducing a disinfecting amount of a disinfectant comprises separatelyintroducing in separate locations two different solutions comprisingperacetic acid.